There are more than 300,000 species of plants. They show a wide diversity of forms, ranging from delicate liverworts, adapted for life in a damp habitat, to cacti, capable of surviving in the desert. The plant kingdom includes herbaceous plants, such as corn, whose life cycle is measured in months, to the giant redwood tree, which can live for thousands of years. This diversity reflects the adaptations of plants to survive in a wide range of habitats. This is seen most clearly in the flowering plants (phylum Angiospermophyta), which are the most numerous, with over 250,000 species. They are also the most widespread, being found from the tropics to the arctic.
The process of plant breeding involving man's intervention in natural breeding and selection is some 20,000 years old. It has produced remarkable advances in adapting existing species to serve new purposes. The world's economics was largely based on the successes of agriculture for most of these 20,000 years.
Plant breeding involves choosing parents, making crosses to allow recombination of gene (alleles) and searching for and selecting improved forms. Success depends on the genes/alleles available, the combinations required and the ability to create and find the correct combinations necessary to give the desired properties to the plant. Molecular genetics technologies are now capable of providing new genes, new alleles and the means of creating and selecting plants with the new, desired characteristics.
Plants specifically improved for agriculture, horticulture and other industries can be obtained using molecular technologies. As an example, great agronomic value can result from modulating the size of a plant as a whole or of any of its organs. The green revolution came about as a result of creating dwarf wheat plants, which produced a higher seed yield than taller plants because they could withstand higher levels and inputs of fertilizer and water.
Similarly, modulation of the size and stature of an entire plant, or a particular portion of a plant, allows production of plants better suited for a particular industry. For example, reductions in the height of specific ornamentals, crops and tree species can be beneficial by allowing easier harvesting. Alternatively, increasing height may be beneficial by providing more biomass. Other examples of commercially desirable traits include increasing the length of the floral stems of cut flowers, increasing or altering leaf size and shape, enhancing the size of seeds and/or fruits, enhancing yields by specifically stimulating hormone (e.g. Brassinolide) synthesis and stimulating early flowering or evoking late flowering by altering levels of gibberellic acid or other hormones in specific cells. Changes in organ size and biomass also result in changes in the mass of constituent molecules such as secondary products. To summarize, molecular genetic technologies provide the ability to modulate and manipulate growth, development and biochemistry of the entire plant as well as at the cell, tissue and organ levels Thus, plant morphology, development and biochemistry are altered to maximize or minimize the desired plant trait.